First stereoselective synthesis of the versatile chiral building block (7aR)-5,6-dihydro-7a-methyl-1H-indene-2,7(4H,7aH)-dione

Article abstract of DOI:10.1055/s-2000-8196

A versatile chiral building block (7aR)-5,6-dihydro-7a-methyl-1H-indene-2,7(4H,7aH)-dione (4) was firstly enantiomerically synthesized from the microbial transformation ketol product 6 in 61.3% overall yield and over 96% ee.

Full text of DOI:10.1055/s-2000-8196

PAPER
1673
First Stereoselective Synthesis of the Versatile Chiral Building Block
(7aR)-5,6-Dihydro-7a-methyl-1H-indene-2,7(4H,7aH)-dione
Zhi-Liang Wei, Zu-Yi Li,* Guo-Qiang Lin*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China
Fax 86(21)64166128; E-mail: lizy@pub.sioc.ac.cn
Received 1 June 2000; revised 27 July 2000
form,10 baker’s yeast reduction of 5 provided a nearly
equivalent mixture of diastereomeric ketols 6 and 7.8
However, the highly reductive fungus, Geotrichum sp.,11
could reduce the diketone 5 more efficiently to give the
ketols 6 and 7 in the ratio of 83:17 (Scheme 1).
Abstract: A versatile chiral building block (7aR)-5,6-dihydro-7a-
methyl-1H-indene-2,7(4H,7aH)-dione (4) was firstly enantiomeri-
cally synthesized from the microbial transformation ketol product 6
in 61.3% overall yield and over 96% ee.
Key words: asymmetric synthesis, microbial reduction, ketols, bi-
cyclic compounds, Wieland Miescher ketone analog
Enantiomerically pure bicyclic enones 1 4 may consti-
tute useful building blocks for elaboration of synthetic
strategies in the field of steroids, terpenes and related mol-
4
ecules.1
The enantiomerically enriched forms of
Wieland Miescher ketone 1 and Hajos Parrish ketone 2
could be efficiently prepared by asymmetric cyclization
of their prochiral precursors, 2-methyl-2-(3-oxobutyl)-
1,3-cyclohexanedione and 2-methyl-2-(3-oxobutyl)-1,3-
cyclopentanedione, respectively, by the use of proline-
enamine2 or catalytic antibody.3 However, ring construc-
tion of the fused cyclopentenones 3 and 4 from their cor-
responding 1,3-cycloalkanedione derivatives using the
standard aldol condition is quite difficult.4 Though opti-
cally active 3 has been asymmetrically synthesized by
Trost5 and Brooks,6 preparation of the enantiomerically
enriched form of 4 has not been reported so far, to our
knowledge.7 In this paper, we present the first enantiose-
lective synthesis of the versatile building block 4 from the
ketol 6.
Scheme 1
Because the two diastereomeric ketols 6 and 7 could not
be separated efficiently by silica gel chromatography, the
mixture obtained from microbial reduction of 5 was con-
verted to the corresponding acetals 8 and 9, which could
be separated efficiently by chromatography. After treat-
ment with 3 M HCl, compounds 8 and 9 were converted
to ketols 6 and 7 in nearly optically pure forms, respec-
tively (Scheme 2).
Reagents and conditions: i) Cat. p-TsOH, HOCH2CH2OH, CH(OEt)3;
ii) 3 M HCl.
Scheme 2
Enantiomerically pure ketol 6 could be prepared by mi-
croorganism asymmetric reduction of the corresponding
prochiral diketone 5,8 or by enzymatic hydrolysis of
prochiral dienol diacetate of 5 developed recently by Re-
nouf et al.9 Though baker’s yeast mediated asymmetric re-
duction of ketones has been widely used to obtain chiral
building blocks since it is cheap, versatile, and easy to per-
Considering that the hydroxy group would interfere with
the subsequent transformations, it was efficiently protect-
ed as the tert-butyldimethylsilyl ether 10. Wacker oxida-
tion of the olefin provided the corresponding dione 11
with good yield (83%). Cyclization and dehydration of di-
Synthesis 2000, No. 12, 1673–1676 ISSN 0039-7881 © Thieme Stuttgart · New York

1674
Z.-L. Wei et al.
PAPER
one 11 to bicyclic enone 12 was smoothly achieved in
high yield (91%) by using potassium tert-butoxide in tert-
butyl alcohol at room temperature. Deprotection of the si-
lyl ether with aqueous HF gave the bicyclic ketol 13 effi-
ciently (90%), which was subjected to Swern oxidation to
provide the desired enedione 4 in 94% yield (Scheme 3).
Therefore,
(7aR)-5,6-dihydro-7a-methyl-1H-indene-
2,7(4H,7aH)-dione (4) was enantiomerically synthesized,
for the first time, from the microbial transformation ketol
product 6 in 61.3% overall yield.
Scheme 4
on a Finnigan MAT-4021 instrument or a Kratos Concept 1H Series
1
Mass Spectromer. H NMR spectra were recorded on a Bruker
Am300 (300 MHz) or Bruker DRX-400 (400 MHz) spectrometer
with TMS as the internal standard. Optical rotation were measured
on a Perkin Elmer 241 polarimeter. Chiral HPLC was carried out
using Chiralpak AD column (0.46 cm x 25 cm) detected at UV 254
nm, flow-rate: 0.7 mL/min. TLC was carried out using HSG F254 sil-
ica gel plates and silica gel (200 400 mesh) was used for chroma-
tography. Organic extracts were dried over anhyd. Na2SO4.
Bioreduction of 2-Methyl-2-(2-propenyl)cyclohexane-1,3-dione
(5)
Wet mycelium of Geotrichum sp. (15 g) was suspended in 5% glu-
cose solution (50 mL) and the dione 5 (1 mmol) was added slowly.
The mixture was shaken at 30 °C. After completion of the biotrans-
formation (15 h), the mycelium was filtered out and washed with
EtOAc. The filtrate was saturated with NaCl and extracted with
EtOAc (4 x 50 mL). The combined extracts were washed with brine,
dried, filtered, and evaporated under reduced pressure. The residue
was purified by chromatography to provide a mixture of the ketols
6 and 7 (87%) in the ratio of 83:17, determined by GC-MS analysis.
Reagents and conditions: i) TBDMSCl, DMF, imidazole, DMAP,
60 °C, 24 h, 96%; ii) PdCl2, CuCl, DMF, H2O, O2, r.t., 30 h, 83%; iii)
t
tBuOK, BuOH, r.t., 4 h, 91%; iv) aq HF (40%), CH3CN, r.t., 48 h,
90%; v) Swern oxidation, 94%.
Ethylene Acetals 8 and 9
Scheme 3
A solution of 6/7 (1.5 g, 9 mmol), ethylene glycol (3 g, 48 mmol),
triethyl orthoformate (4.8 g, 18 mmol), and p-toluenesulfonic acid
(200 mg, 1.2 mmol) was stirred at r.t. for 20 h. Sat. NaHCO3
(40 mL) was added and the mixture was extracted with Et2O
(3 x 50 mL). The organic layers were combined and washed with
brine, dried, filtered, and evaporated under reduced pressure. The
residue was purified by silica gel chromatography (petroleum ether/
EtOAc, 20:1) to provide 8 (1.50 g, 81%) and 9 (0.35 g, 16%).
It is worth mentioning that the silyl ether 12 was initially
treated with Bu4NF and anhydrous THF in the deprotec-
tion step, and unexpectedly a mixture of the diastereomer-
ic bicyclic ketol 13 and 14 was obtained in 3:1 ratio.
Chiral HPLC analysis indicated that the major diastereoi-
somer 13 was in 49% ee, while the other 14 was in nearly
racemic forms. This was thought to be a result of carbon-
carbon bonds cleavage and reformation, under the depro-
tection conditions, through the intermediates 15 to 17 due
(1S,2S)-3-(1,3-Dioxolan-2-yl)-2-methyl-2-(2-propenyl)cyclo-
hexan-1-ol (8)
Colorless oil; [ ]D
21
4.7 (c 1.0, CHCl3).
to the unstable bicyclic ketol anion 15. However, the ketol IR (film):
13 obtained under the conditions was still in enantiomeri-
= 3500, 2920, 1640, 1450, 1410, 1120, 1070, 910
max
1
cm .
1H NMR (300 MHz, CDCl3): = 5.98 5.83 (m, 1H), 5.12 5.05 (m,
2H), 4.04 3.87 (m, 4H), 3.61 (s, 1H), 3.20 (br s, 1H), 2.54 (dd, 1H,
J = 13.2, 6.8 Hz), 2.01 (dd, 1H, J = 13.2, 8.1 Hz), 1.90 1.51 (m,
6H), 1.01 (s, 3H).
EIMS: m/z (rel. intensity) = 212 (M+, 0.6), 194 (4.3), 108 (51.1), 99
(100), 86 (72.3), 55 (33.8), 43 (34.2), 41 (46).
cally enriched form. It might be either due to absorption
of moisture from outside or that some oxygen anion inter-
mediate 15 still did not racemize when the reaction was
worked up (Scheme 4). As a result, nearly optically pure
13 (>96% ee) determined by chiral HPLC analysis was
provided as the sole diasteromeric product using aqueous
HF as the deprotection conditions.
Anal. Calcd. for C12H20O3: C, 67.89; H, 9.50. Found: C, 67.48; H,
9.72.
All mps are uncorrected. IR spectra were recorded on a Shimadzu
IR-440 spectrometer. EIMS were run on an HP-5989A mass spec-
trometer and high resolution mass spectra (HRMS) were recorded
(1S,2R)-3-(1,3-Dioxolan-2-yl)-2-methyl-2-(2-propenyl)cyclo-
hexan-1-ol (9)
Colorless oil; [ ]D
21
22.8 (c 0.9, CHCl3).
Synthesis 2000, No. 12, 1673–1676 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
First Stereoselective Synthesis of the Versatile Chiral Building Block
1675
IR (film): max = 3400, 2920, 1700, 1640, 1450, 1180, 1120, 1050,
950, 910 cm .
Anal. Calcd. for C16H30O3Si: C, 64.38; H, 10.13. Found: C, 64.51;
H, 10.43.
1
1H NMR (300 MHz, CDCl3): = 6.00 5.85 (m, 1H), 5.15 5.00 (m,
2H), 4.05 3.85 (m, 4H), 3.72 (dd, 1H, J = 5.1, 2.4 Hz), 2.90 (br s,
1H), 2.45 2.25 (m, 2H), 1.80 1.50 (m, 6H), 1.03 (s, 3H).
EIMS: m/z (rel. intensity) = 212 (M+, 1.9), 195 (12), 108 (42), 99
(100), 86 (46.4), 55 (43.2), 43 (51.7), 41 (77.5).
(7S,7aR)-7-[(tert-Butyldimethylsilyl)oxy]-5,6-dihydro-7a-meth-
yl-1H-inden-2(4H,7aH)-one (12)
To a solution of the dione 11 (850 mg, 2.85 mmol) in tert-butyl al-
cohol (20 mL) was added potassium tert-butoxide (320 mg,
2.85 mmol), and the reaction mixture was stirred under N2 for 4 h.
The solution was then mediated to pH 7.0 with 0.5 M HCl at 0 °C,
and evaporated under reduced pressure to remove the alcohol. The
residue was dissolved in EtOAc (150 mL) and washed with sat.
brine, dried, filtered, and condensed. The crude product was puri-
fied by chromatography over silica gel (petroleum ether/EtOAc,
5:1) to provide 12 (725 mg, 91%) as a white solid, mp: 81 82 °C.
21
Anal. Calcd. for C12H20O3: C, 67.89; H, 9.50. Found: C, 67.58; H,
9.75.
(2S,3S)-3-Hydroxy-2-methyl-2-(2-propenyl)cyclohexan-1-one
(6)
A mixture of the ethylene acetal 8 (1.4 g, 6.6 mmol) and 3 M HCl
(40 mL) was stirred at r.t. for 10 h. The reaction mixture was ex-
tracted with CH2Cl2 (3 x 50 mL). The organic layers were combined
and washed with sat. brine, dried, filtered, and condensed. The res-
idue was purified by chromatography on silica gel (petroleum ether/
EtOAc, 5:1) to provide the ketol 6 (1.1 g, 98%), whose spectral data
were identical with those reported.8
[ ]D
121 (c 1.0, CHCl3).
IR (KBr):
790 cm .
= 2950, 1700, 1630, 1470, 1280, 1090, 1030, 850,
max
1
1H NMR (300 MHz, CDCl3): = 5.71 (d, 1H, J = 1.5 Hz), 3.74 (s,
1H), 2.53 and 1.81 (AB, 2H, J = 18.0 Hz), 2.53 2.47 (m, 1H),
2.35 2.24 (m, 1H), 1.92 1.51 (m, 4H), 1.16 (s, 3H), 0.72 (s, 9H),
0.07 (s, 3H), 0.09 (s, 3H).
13C NMR (75 MHz, CDCl3): = 208.06, 184.52, 127.84, 74.50,
48.38, 46.48, 28.34, 27.02, 25.54, 23.78, 21.02, 17.80, 4.46, 5.48.
(2S,3S)-3-[(tert-Butyldimethylsilyl)oxy]-2-methyl-2-(2-prop-
enyl)cyclohexan-1-one (10)
A mixture of ketol 6 (770 mg, 4.6 mmol), DMF (5 mL), imidazole
(1 g, 15 mmol), 4-(dimethylamino)pyridine (130 mg, 1 mmol), and
tert-butyldimethylsilyl chloride (1.12 g, 7.5 mmol) was stirred un-
der N2 at 60 C for 24 h. After cooling the mixture, Et2O (150 mL)
was added. The organic layer was washed with sat. brine, dried, fil-
tered, and evaporated under reduced pressure. The residue was pu-
rified by chromatography over silica gel (petroleum ether/EtOAc,
10:1) to provide silyl ether 10 (1.25 g, 96%).
Anal. Calcd. for C16H28O2Si: C, 68.52; H, 10.06. Found: C, 68.38;
H, 10.30.
(7S,7aR)-5,6-Dihydro-7-hydroxy-7a-methyl-1H-inden-
2(4H,7aH)-one (13)
A solution of silyl ether 12 (140 mg, 0.5 mmol) in MeCN (2 mL)
and 40% of aqueous HF (70 L, 1.5 mmol) was stirred at r.t. for two
days. The solution was mediated to neutral with sat. NaHCO3 and
extracted with EtOAc (3 x 15 mL). The combined organic layers
were dried, filtered, condensed, and the obtained residue was puri-
fied over silica gel (petroleum ether/EtOAc, 1:1) to provide 13
25
Colorless oil; [ ]D
54 (c 1.0, CHCl3).
IR (film):
800 cm .
= 2980, 1730, 1480, 1390, 1280, 1110, 1030, 870,
max
1
1H NMR (300 MHz, CDCl3): = 5.80 5.60 (m, 1H), 5.07 5.00 (m,
2H), 3.70 (dd, 1H, J = 8.1, 4.0 Hz), 2.50 (dd, 1H, J = 14.3, 7.8 Hz),
2.39 2.30 (m, 3H), 2.04 1.80 (m, 3H), 1.63 1.58 (m, 2H), 1.09 (s,
3H), 0.9 (s, 9H), 0.07 (s, 6H).
24
(75 mg, 90%) as a white solid, mp: 139 140 °C; 96.2% ee; [ ]D
64 (c 1.0, CHCl3). The enantiomeric purity was determined by
HPLC analysis using Chiralpak AD column (eluent, hexane/i-
PrOH, 95:5), detected at UV254
.
EIMS: m/z (rel. intensity) = 265 (1.22), 225 (16.3), 183 (28.8), 133
(25.2), 75 (100), 73 (50.8), 57 (38.3), 41 (74).
IR (KBr): max = 3450, 2940, 1680, 1610, 1410, 1290, 1230, 1140,
1
1055, 980, 890 cm .
Anal. Calcd. for C16H30O2Si: C, 68.03; H, 10.70. Found: C, 67.83;
H, 10.73.
1H NMR (300 MHz, CDCl3): = 5.89 (d, 1H, J = 1.6 Hz), 3.92
3.90 (m, 1H), 2.84 and 2.00 (AB, 2H, J = 18.4 Hz), 2.69 2.63 (m,
1H), 2.49 2.38 (m, 1H), 2.20 (br s, 1H), 2.10 1.96 (m, 1H), 1.88
1.74 (m, 3H), 1.29 (s, 3H).
13C NMR (75 MHz, CDCl3): = 209.31, 185.45, 128.18, 73.49,
48.21, 46.13, 28.18, 27.07, 24.10, 20.94.
EIMS: m/z (rel. intensity) = 166 (M+, 20), 148 (19), 138 (30.6), 123
(47.9), 110 (100), 109 (51), 95 (61.1), 79 (62.3), 67 (35.28), 41
(37.21).
(2S,3S)-3-[(tert-Butyldimethylsilyl)oxy]-2-methyl-2-(2-oxopro-
pyl)cyclohexan-1-one (11)
To a solution of 10 (1.25 g, 4.5 mmol) in DMF (3 mL) was added
palladium chloride (60 mg, 0.33 mmol), cuprous chloride (450 mg,
4.5 mmol) and H2O (0.5 mL). The reaction mixture was stirred un-
der an O2 atm at r.t. for 30 h. EtOAc (100 mL) was added to the re-
action mixture and filtered. The filtrate was washed with sat. brine,
dried, filtered, and evaporated under reduced pressure. The obtained
residue was purified by chromatography over silica gel (petroleum
ether/EtOAc, 10:1) to afford 11 (1.1 g, 83%).
HRMS: calcd. for (C10H14O2)+: 166.0994. Found: 166.0990.
(7aR)-5,6-Dihydro-7a-methyl-1H-indene-2,7(4H,7aH)-dione (4)
To a solution of oxalyl chloride (0.14 mL, 1.64 mmol) in CH2Cl2
(3 mL) was added dropwise at 78 °C DMSO (0.3 g, 3.79 mmol).
After stirring for 0.5 h, a solution of 13 (220 mg, 1.33 mmol) in
CH2Cl2 (3 mL) was added, and the mixture was stirred at 60 °C
for 2 h. Then a solution of Et3N (3 mL) and CH2Cl2 (5 mL) was add-
ed. The mixture was warmed slowly to r.t. and poured into cooled
H2O (20 mL), extracted with CH2Cl2 (3 x 30 mL). The organic lay-
ers were combined and washed with brine, dried, filtered, and con-
densed. The residue was purified by chromatography over silica gel
(petroleum ether/EtOAc, 2:1) to afford the desired 4 (205 mg, 94%).
20
25
Colorless oil; [ ]D
2.3 (c 2.0, CHCl3).
IR (film):
= 2950, 1710, 1460, 1360, 1250, 1080, 1020, 840,
max
1
780 cm .
1H NMR (400 MHz, CDCl3): = 4.14 (d, 1H, J = 4.9 Hz), 2.90 and
2.67 (AB, 2H, J = 17.8 Hz), 2.55 2.40 (m, 1H), 2.30 2.23 (m, 1H),
2.10 (s, 3H), 2.10 1.90 (m, 2H), 1.75 1.60 (m, 2H), 1.25 (s, 3H),
0.83 (s, 9H), 0.01 (s, 3H), 0.05 (s, 3H).
13C NMR (100 MHz, CDCl3): = 213.11, 207.83, 77.05, 60.38,
53.89, 45.18, 37.13, 31.61, 28.40, 25.80, 20.82, 20.35, 18.06, 14.21,
4.33, 5.14.
[ ]D
47.3 (c 0.42, CHCl3).
Synthesis 2000, No. 12, 1673–1676 ISSN 0039-7881 © Thieme Stuttgart · New York

1676
Z.-L. Wei et al.
PAPER
1
IR (film): max = 2962, 1712, 1621 cm .
Zhong, G.; List, B.; Shabat, D.; Anderson, J.; Gramatikova,
S.; Lerner, R. A.; Barbas, C. F., III. J. Am. Chem. Soc. 1998,
120, 2768.
1H NMR (300 MHz, CDCl3): = 5.72 (d, 1H, J = 1.5 Hz), 3.10 and
2.05 (AB, 2H, J = 18.9 Hz), 2.84 2.61 (m, 3H), 2.39 2.31 (m, 1H),
2.25 2.15 (m, 1H), 1.73 1.56 (m, 1H), 1.45 (s, 3H).
13C NMR (75 MHz, CDCl3): = 209.28, 205.83, 181.95, 127.05,
58.63, 43.63, 36.77, 26.20, 25.58, 24.76.
EIMS: m/z (rel. intensity) = 164 (M+, 100), 136 (62.5), 121 (74.9),
108 (42.7), 93 (31.1), 80 (60.9), 79 (79.5), 77 (27.5).
(4) Mori, M.; Isono, N.; Kaneta, N.; Shibasaki, M. J. Org. Chem.
1993, 58, 2972.
Dauben, W. G.; Hart, D. J. J. Org. Chem. 1977, 42, 3787.
(5) Trost, B. M.; Curran, D. P. J. Am. Chem. Soc. 1980, 102, 5699.
Trost, B. M.; Curran, D. P. Tetrahedron Lett. 1981, 22, 4929.
Trost, B. M.; Curran, D. P. J. Am. Chem. Soc. 1981, 103, 7380.
(6) Brooks, D. W.; Grothaus, P. G.; Irwin, W. L. J. Org. Chem.
1982, 47, 2820.
HRMS: calcd. for (C10H12O2)+: 164.08373. Found: 164.08428.
Brooks, D. W.; Woods, K. W. J. Org. Chem. 1987, 52, 2036.
(7) For the preparation of racemic 4, see references 4 and
Kinoshita, A.; Mori, M. Chem. Lett. 1994, 1475.
Yanami, T.; Ballatore, A.; Miyashita, M.; Kato, M.;
Yoshikoshi, A. Synthesis 1980, 407.
References
(1) Fuhshuku, K.; Funa, N.; Akeboshi, T.; Ohta, H.; Hosomi, H.;
Ohba, S.; Sugai, T. J. Org. Chem. 2000, 65, 129; and the
references therein.
Altenbach, H. -J. Angew. Chem., Int. Ed. Engl. 1979, 18, 940.
Bestmann, H. J.; Schade, G.; Lutke, H.; Monius, T. Chem.
Ber. 1985, 118, 2640.
Isaacs, R. C. A.; Di Grandi, M. J.; Danishefsky, S. J. J. Org.
Chem. 1993, 58, 3938.
Arseniyadis, S.; Yashunsky, D. V.; de Freitas, R. P.; Dorado,
M. M.; Potier, P.; Toupet, L. Tetrahedron 1996, 52, 12443.
Van Gool, M.; Zhao, X. -Y.; Sabbe, K.; Vandewalle, M. Eur.
J. Org. Chem. 1999, 2241.
(8) Brooks, D. W.; Mazdiyasni, H.; Grothaus, D. G. J. Org.
Chem. 1987, 52, 3223.
(9) Renouf, P.; Poirier, J. M.; Duhamel, P. J. Chem. Soc., Perkin
Trans 1 1997, 1739.
Trost, B. M. Acc. Chem. Res. 1980, 13, 385.
Emori, E.; Iida, T.; Shibasaki, M. J. Org. Chem. 1999, 64,
5318.
Srikrishna, A.; Reddy, T. J. Tetrahedron 1998, 54, 11516.
Srikrishna, A.; Nagaraju, S.; Reddy, T. J.; Venkateswarlu, S.
Pure Appl. Chem. 1996, 68, 699.
Renouf, P.; Poirier, J. M.; Duhamel, P. J. Org. Chem. 1999,
64, 2513.
(10) Csuk, R.; Glanzer, B. I. Chem. Rev. 1991, 91, 49.
Sevi, S. Synthesis 1990, 1.
Santaniello, E.; Ferraboschi, P.; Grisenti, P.; Manzocchi, A.
Chem. Rev. 1992, 92, 1071.
(2) Eder, U.; Sauer, G.; Wiechert, R. Angew. Chem., Int. Ed. Engl.
1971, 10, 496.
(11) Wei, Z. -L.; Li, Z. -Y.; Lin, G. -Q. Bioorg. Med. Chem. 2000,
8, 1129.
Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615.
Gutzwiller, J.; Buchschacher, P.; Furst, A. Synthesis 1977,
167.
Wei, Z.; Li, Z.; Lin, G.; Tang, M.; Xia, L. Sci. China, Ser. B
2000, 43, 331.
Buchschacher, P.; Furst, A. Org. Synth. 1985, 63, 37.
Hajos, Z. G.; Parrish, D. R. Organic Synthesis; Wiley & Sons:
New York, 1990; VII, 368.
Article Identifier:
1437-210X,E;2000,0,12,1673,1676,ftx,en;F02900SS.pdf
(3) Zhong, G.; Hoffmann, T.; Lerner, R. A.; Danishefsky, S.;
Barbas, C. F., III. J. Am. Chem. Soc. 1997, 119, 8131.
Synthesis 2000, No. 12, 1673–1676 ISSN 0039-7881 © Thieme Stuttgart · New York